1. Technical Field
The present disclosure relates to amplifiers having a plurality of amplifier stages, and in particular to methods and structures for detecting the output power of such amplifiers.
2. Discussion of the Related Art
Power detectors are used in radio frequency (RF) communications systems to monitor the power of an RF signal that is output to an antenna. The power detector produces a direct current (DC) signal that is proportional to the power of the RF signal being sampled. The communications system can then use the DC signal as a measure of the power of the RF signal being transmitted, and can make adjustments in order to maintain the output power within system specifications.
Conventional power detection techniques sample the RF signal immediately before the antenna, after a final amplifier stage of a multi-stage amplifier amplifies the RF signal and after the RF signal passes through a final matching network between the final amplifier stage and the antenna. However, under conditions where the amplifier load is not impedance matched properly, which may be due to a faulty or broken antenna or to environmental conditions, the voltage monitored by the power detector can be in error, in that the voltage no longer accurately predicts the true output power. Depending upon the conditions, the detector could overestimate or underestimate the actual power.
An underdetection of the output power could have serious consequences. For instance, the communications system may work to increase the output power by further amplifying the RF signal, based on the erroneous information that the output power is too low. The communications system might then increase the output power beyond a safe or regulated level. Outputting too much power could lead to a violation of health regulations, a danger to users, lawsuits, and the like. In addition, the efficiency of the communications system would be reduced, since the communications system would be expending more energy than necessary to amplify the outgoing RF signal. Such would be particularly problematic in wireless applications, such as cellular phones, that operate on battery power. The battery power reserve could be needlessly depleted.
Therefore, there exists a need to accurately measure the power of an RF signal amplified by a multi-stage amplifier, and to avoid underdetecting the power of the RF signal.